The present invention relates to an encoder circuit for converting a thermometric code into a binary code and, particularly, to an improved encoder circuit for converting the thermometric code into the binary code at a high speed and directly.
As is well known, an analog-digital (hereinafter abbreviated as A/D) converter is a device that converts an analog signal into a digital signal. There have heretofore been known A/D converters of a variety of kinds. Among them, a flush A/D converter features a short conversion time since it effects a comparison operation only a single time. In the flush A/D converter, a reference voltage is divided into a plurality of voltages by a plurality of resistors, voltages at dividing points are compared with a voltage of an analog input signal at the same time, and an address of the dividing point having a voltage closest to the voltage of the analog input signal is output as a digital signal. Therefore, comparators are required in a number equal to the number of the dividing points, resulting in an increase in the scale of the circuit.
The flush A/D converter requires a code converter for converting the code delivered from the above-mentioned plurality of comparators (hereinafter referred to as xe2x80x9ccomparator arrayxe2x80x9d) into the binary code. In the case of an N-bit flush A/D converter, the output of the comparator array if the code having a (2Nxe2x88x921)-bit length. This code is a digital code expressing the voltage of an analog voltage signal, which, in this field of art, is called xe2x80x9cthermometric codexe2x80x9d and is abbreviated as xe2x80x9cTHCxe2x80x9d. Therefore, the above code conversion circuit is called a thermometric-binary code conversion circuit. An ideal thermometric code is the one in which consecutive xe2x80x9c1xe2x80x9ds precede and xe2x80x9c0xe2x80x9ds succeed. A transition point from xe2x80x9c1xe2x80x9d into xe2x80x9c0xe2x80x9d represents the level of the analog voltage signal.
The thermometric-binary code conversion circuit of this type has been disclosed in, for example, JP-H8-36466 (hereinafter referred to as xe2x80x9cprior art 1xe2x80x9d). This thermometric-binary code conversion circuit employs a dichotomizing search to decrease a conversion error and, hence, converts the thermometric code directly into the binary code without forming an intermediate code.
As will be described later in detail, the thermometric-binary code conversion circuit involves problems as described below.
A first problem is that the rate of conversion decreases in proportion to an increase in the number of the bits that are to be converted.
A second problem is that the number of elements to be used increases with an increase in the number of the bits that are to be converted; i.e., a conspicuously increased area is occupied by the elements and the consumption of electric power increases greatly.
The object of the present invention therefore is to provide a thermometric-binary code conversion method and a circuit capable of increasing the rate of conversion, decreasing an area occupied by the elements and decreasing the consumption of electric power without losing error correction function.
The thermometric-binary code conversion circuit according to the present invention realizes a code conversion using a logic circuit that realizes a thermometric-binary code equation expressed by Boolean algebra. Concretely, the thermometric-binary code conversion circuit has a conversion unit for directly converting a thermometric code into a binary code by using a multi-input-three-value-output logic circuit for a logical product portion of the thermometric-binary code conversion equation, without having dependence between a high-order bit and a low-order bit of the binary code that is to be output, eliminating the need for holding the data of high-order bits, without forming an intermediate code and without losing the error-decreasing function.
The thermometric-binary code conversion circuit according to the present invention is capable of determining the values of all output bits through a single time of operation without having dependence between the high-order bit and the low-order bit of the binary code that is to be output, and suppresses an increase in the delay time necessary for the code conversion caused by an increase in the number of the output bits.
Further, the thermometric-binary code conversion circuit according to the present invention suppresses a phase difference among the output bits and, hence, requires neither the output latch circuit for holding the data nor the flip-flop circuit and, further, makes it possible to decrease the area occupied by the elements and to decrease the consumption of electric power.